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That would probably be the contrast-detection autofocus. Unlike the advanced phase-shift detection autofocus normally used for still shot AF in DSLRs, contrast-detection AF has to do a bit of "hunting" to find the point of highest contrast.

The use of contrast AF is actually not limited to point and shoot cameras, it is also used in DSLRs when using a live view mode. So as long as you are actively focusing, there is probably nothing you can do to prevent momentary focus hunting like that (I checked the manual, and there did not seem to be anything about AF during video that might help.)